The field of the invention is microwave resonators, and particularly, resonators employed in electron paramagnetic resonance spectroscopy.
Electron paramagnetic resonance spectroscopy is conducted to study electrons which are in a paramagnetic state and which is called electron paramagnetic resonance (EPR) or electron spin resonance (ESR). In electron paramagnetic resonance spectroscopy a sample to be investigated is subjected to a polarizing magnetic field and one or more radio frequency magnetic fields. The frequency, strength, direction, and modulation of the applied magnetic fields varies considerably depending upon the particular phenomena being studied. Apparatus such as that disclosed in U.S. Pat. Nos. 3,358,222 and 3,559,043 have been employed for performing such experiments in laboratories. Samples which are the subject of the EPR measurement are placed in a microwave resonator where they are subjected to the RF magnetic field. The microwave resonator may take the form of a cavity resonator such as that disclosed in U.S. Pat. Nos. 3,931,569 and 3,757,204, or it may be a loop-gap resonator such as that disclosed in U.S. Pat. No. 4,446,429. A major objective of the resonator is to apply a uniform RF magnetic field throughout the extent of the sample.
Loop-gap resonators (LGR) have become a preferred resonator geometry for experiments at frequencies below X band. Cavity resonators are generally preferred at higher frequencies to about 100 GHz, with Fabry-Perot resonators preferred at ultrahigh frequencies. Both LGRs and cavity resonators are in common use at X-band (10 GHz), Q-band (35 GHz) and S-band (3 GHz), which are by far the most widely used frequency for EPR experiments. The reason for these preferences is primarily convenience. Cavity resonators are awkwardly large at S band, LGRs become extremely small at Q band, and cavity resonators are, in turn, too small to handle easily at ultrahigh frequencies.
A benefit of LGRs is that the length to diameter ratio of the sample-containing loop is typically about five, resulting in a relatively uniform microwave field over the sample. This is a substantial benefit in experiments using line samples that extend through the resonator, since all portions of the sample respond in the same way to the incident microwave field. For cavity resonators on the other hand, the microwave field varies cosinusoidally over the sample, with the number of half cycles of variation determined by the selected index of the microwave resonant modexe2x80x94usually one half cycle.
The present invention is a resonator for use in applications where a highly uniform RF magnetic field is desired along an axial dimension. More specifically, the resonator includes a central cavity section having cross-sectional dimensions set to establish a cutoff condition for a selected RF wave propagation mode and frequency; and a pair of uniform-field supporting end sections connected to each end of the central cavity section. It has been discovered that when the central cavity section is operated at the cutoff frequency for a transverse electric propagation mode, the RF fields produced along the entire axial length of the central cavity section are substantially uniform regardless of its length. A sample placed in the central cavity section and disposed along its axis is thus subjected to a substantially uniform RF magnetic field.
A general object of the invention is to apply a substantially uniform RF field to a sample during an EPR experiment. The sample may be contained in a tube or a cuvette which is mounted along the axis of the central cavity section. The sample is subjected to substantially the same RF magnetic field along its entire axial length, regardless of its length.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.